The Sign of the Six
by renrenren3
Summary: Sherlock is on the trail of a killer and ends up crossing paths with a certain American detective.


Written for a Sherlock exchange where the recipient asked for a crossover and I thought it'd be cool to have Sherlock and Castle meet. I wanted to turn this into a full case, loosely inspired on the _Sign of the Four_ book, but I ran out of time.

-x-

"There's nothing we can do," Lestrade said. "Gut feeling is not enough to arrest someone."

John's gaze went from the inspector to his friend. The atmosphere at 221b had been tense lately, and Lestrade's decision to close the case had only made Sherlock more irritable than ever.

"It's not gut feeling," Sherlock replied. "I know he's guilty, and I know he'll kill again."

"Then give me something," Lestrade said. "I can't get a warrant without any proof."

Sherlock snorted and started pacing the floor. "He's far too smart to have left any incriminating evidence behind."

"Then what do you want me to do?" Lestrade snapped. "He's not even in the official suspects' list. By now he'll be halfway to the United States..."

"Right!" Sherlock exclaimed. He stopped pacing abruptly and wheeled around, clasping his hands on John's shoulders. John raised his eyebrows at the sudden change in Sherlock's attitude.

"As you said, inspector, there's nothing _you_ can do," Sherlock grinned. "John, pack your things. We're going to New York."

-x-

"What do we have? Another murder?" Castle asked as Beckett sped through the evening traffic. He shot her an alarmed look. Usually she was a much more careful driver than this. "Oooh," he squealed, hit by a sudden idea. "It's a serial killer!"

He rubbed his hands gleefully and was rewarded by another of Beckett's long-suffering sighs. "No, Castle, there's no serial killer."

"Then why are we in such a hurry?" Castle insisted, holding on for dear life as Beckett swerved between two cars and blatantly ignored a red light.

Beckett looked sideways at him. "We just got a call about a break-in on 3rd Avenue," she said with just the barest hint of a smirk.

Castle pulled a face. "A break-in?" he said in the same tone he'd used on a previous occasion to say things like "A fourth Indiana Jones movie?"

"Everyone knows you can't write a book on a break-in," Castle continued, shaking his head. "It's a basic rule of mystery writers."

"Believe it or not, Castle," Beckett said. "We're not your personal research team."

He pouted. "Why does a homicide detective get a call about a break-in, anyway?"

"That," Beckett replied with a smirk, "is where it gets interesting." She paused for a moment, and Castle was sure she was going for a dramatic effect here. "The owner of the flat that was broken into is a certain Mr Alan Jones."

Castle's eyebrows shot up. "Alan Jones," he repeated. "That's the victim in our murder case."

"Exactly," she confirmed. "And I'm very curious to find out why someone would break into his home not two days after his death."

"You think this isn't a coincidence," Castle said. It wasn't a question.

Beckett snorted. "I'm thinking we don't have any leads for the Jones case. It might just be a burglar, but..."

"A burglar out and about in broad daylight?" he replied.

"Yes, it's weird," Beckett replied. "Let's hope our man is still inside when we get there."

-x-

"This is a very, very bad idea," John said for the third time in as many minutes.

Sherlock gestured for him to shut up. He was disrupting his trail of thoughts. Sherlock was sure he was on to something here, if only he could find the missing pillow-case...

"Jones was killed only forty-five hours ago," he said, more to himself than for John's benefit. He doubted John would have been able to piece everything together but he liked to lay all the pieces of the puzzle neatly in front of him. "Allowing for time-zone differences, this means that our man might well have done it if he was on the flight before ours."

He crouched down on the floor on all fours, checking under Mr Jones' sofa. Nothing of interest here. From a single glance to the dust he could tell that Mr Jones' housekeeper was currently on holiday and that he'd recently split up with his long-time girlfriend, but that was most probably not relevant to their case.

"Our killed didn't have much time to plan because he knew I was on his trail," he continued, springing back on his feet and moving to examine the books on the coffee table. "It was a rushed job so he's bound to have missed some critical piece of evidence. Something that will allow me to crack this case and tie him inexorably to both murders. Yes, John, what it is?" he said, noticing that John had been gesturing to him for quite a while.

"I heard footsteps," John said. " There's someone outside."

"Of course," Sherlock replied dismissively. "That would be the American police." He went back to the books, all of which displayed a remarkable bad taste and lack of clues.

"The police?" John repeated, trying at the same time to yell and not to be heard. It was futile since the men standing outside knew about their presence already, but Sherlock didn't bother to point it out. John would find out soon enough anyway.

"Yes, the neighbours saw us coming in and called the police," Sherlock said.

"You knew?" John asked. "Why are we still here, we need to get out of this house before they find."

Sherlock huffed. "Calm down, John, you're acting like a criminal."

"If you hadn't noticed, we've just broken into a man's house," John hissed. "This is criminal behaviour!"

Before Sherlock could tell him that he was being unreasonable, two things happened.

First and foremost he figured out where the missing pillow-case was. He was certain that as soon as they got their hands on it this case would be as good as solved.

Also, several police officers broke in yelling, "NYPD, don't move!"

Sherlock thought that they, too, were overreacting.

-x-

Beckett dropped the case files on the table and thumbed through them for some time. Not that she needed a refresher, she knew the details of the Jones case very well. But it always paid off to let the suspects steam a little. The man in front of her might seem calm but the ones who tried to act though were the first to crack.

"Mr Holmes," she said eventually, stapling her hands in front of her. "Your friend told us a very... strange story."

"I thought it was very interesting," Castle interjected. "Just like the plot of a good mystery."

Beckett shot him an annoyed look. "Yes," she said curtly, refusing to be derailed. "But like the plot of a book, it's very unlikely. Maybe you'd care to tell us what's really going on here."

Sherlock Holmes shrugged. "Unlikely, perhaps, but not impossible. As I told you already, I'm a consulting detective and I'm in the process of tracking down a killer."

"Adam Jones' killer," Beckett said, tapping a finger on the relevant file.

"The same man who also killed Jim Robinson," Holmes confirmed.

She did have to resort to her notes for that. "That would have been some weeks ago in England," she said, flipping through the transcript of their earlier interview. "And Scotland Yard, lacking any conclusive evidence, sent you to tail the killed."

"You get props for originality, at any rate," Castle commented.

"We checked and you're not a member of Scotland Yard," Beckett said snapping the case files shut.

Castle's mouth formed an 'o' of mock surprise. "Who would have thought it!"

Holmes ignored him, turning to Beckett instead. "As I said, I'm not officially affiliated with the regular police. I'm a consulting detective. The only one in the world," he added with a touch of pride.

"And you're chasing criminals across two continents out of the goodness of your heart," Beckett quipped.

"Isn't your friend here doing the same?" Holmes replied, nodding to Castle. "He's not affiliated with the police either. He was wearing a bulletproof vest earlier, but it said 'writer' and he wasn't carrying any weapons."

The man turned to address Castle directly. "You're a writer, a mystery writer doing research for a book? But you seem used to be around a crime scene and in an interrogation room. Your clothes are well-tailored and expensive, not something one would wear on a daily basis unless they could afford it."

Castle leaned over to whisper in Beckett's ear. "You've got to hand it to him, he's good," he said in a stage whisper that would have made his mother proud. Beckett rolled her eyes.

Holmes glanced between Beckett and Castle, tapping his fingers on the table. "A successful mystery writer who has done more than enough research for his books but is still following murder cases for no reason at all," he concluded. "Though, that might be incorrect. Perhaps it is not the murders that attract you as much as detective Beckett."

"And this, Mr Holmes, is where you're completely wrong," Beckett replied dryly. "As for the rest, I would have been more impressed if Castle wasn't a world-famous writer. You could have read about him in any newspaper or magazine, his latest book came out in England only last month."

She realized only too late that she'd let it slip that she was up-to-date with Castle's international book releases and started shuffling the papers to cover her confusion.

"You don't have to take my words at face value," Holmes said. "I'm sure that someone will contact you soon to confirm what I just told you."

It wasn't the first time Beckett had heard that excuse. "Unless it's the chief of Scotland Yard that won't be of much help to you," she replied.

"No, I was thinking of the government," Holmes said. "My brother works for them."

Beckett rolled her eyes just as the door to the interrogation room opened. "What?" she snapped, annoyed by the interruption. She blinked in confusion as she realized that Captain Montgomery was standing at the door holding a phone.

"Beckett, you should take this call," he said.

She shot him a questioning look but he simply shrugged and held the phone out to her.

"I'll be back in a minute," she said. "Castle, try not to make a fool of yourself while I'm not here."

-x-

Behind the two-way mirror, Ryan turned to look at Esposito.

"Do you really think this guy is with Scotland Yard?" he asked his partner.

"Please," Esposito snorted. "In all your years in the force, how many times did the suspect turn out to be a secret agent?"

Ryan thought about it. "Once," he replied.

Esposito had tried to forget about that particular case. "All right," he said. "But that's just one time. This guy says he's a British agent sent by Scotland Yard, not even Castle comes up with stuff like that."

"Yeah, you're right," Ryan said. "It would be a good plot for a book though."

Castle seemed to be thinking along the same lines since he was asking the suspect if he was on Her Majesty's secret service.

"Do you think he'll go to England to do some research next?" Ryan asked. "I wouldn't mind if he brought us along."

"I could do with a vacation," Esposito agreed, not paying Ryan much attention. He checked his watch. Beckett's phone call was taking a long time.

"That trick you did, guessing all those thing about me at a glance," Castle was saying. "That was so cool! I'd love to use that in my next book."

Esposito opened the door and peeked into the corridor. Beckett was having a very animated conversation with whoever was on the other side of the phone. Captain Montgomery was still hanging around, looking alternatively at Beckett and at door of the interrogation room.

"Who is she talking with?" he asked Montgomery. "Don't tell me it's really Scotland Yard."

The captain shrugged. "You wouldn't believe if I told you."

"The Queen?" Ryan suggested. He and Esposito laughed, only to stop abruptly when they saw their superior's face. He seemed completely out of his depth.

The three of them turned towards Beckett as soon as she finished the call.

"So?" Montgomery asked.

She sighed. "Sir, I'll need a warrant to search the house of Jones' uncle for a pillow-case," she said. "Ryan, Esposito, you go and tell John Watson that he's free to go."

"What, right now?" Esposito asked. "Who have you been speaking with?"

"The British government," she replied flatly, giving a look to the phone receiver still in her hand.

"His brother really works for the government?" Ryan asked.

"It's more like his brother is the government," Montgomery said.

Before Ryan or Esposito could say anything else, she motioned for them to go back to work.

They heard her say, "Mr Holmes, your brother would like to speak to you," and Holmes' voice rising in indignation.

"No, Mycroft, I did not forget about mummy's Sunday lunch," he was saying.

Captain Montgomery shook his head and walked away.

"Bro," Ryan told Esposito. "That was awesome."

-x-

John didn't like American coffee very much but he still accepted the proffered cup. It was the middle of the night in England, he was tired and hungry and jet-lagged and his brief stint in the holding cells hadn't been exactly thrilling.

"Sorry about arresting you," Detective Ryan said. "But the story you and your friend were telling, it was really unbelievable."

John shrugged. "I don't blame you, Sherlock has that effect on people."

Ryan sat down on a swiveling chair next to John and looked over to the whiteboard, where Sherlock and Detective Beckett were in the middle of a heated discussion that consisted mainly of Sherlock trying to tell Beckett how to do her job.

"Is he really a private detective?" Ryan asked.

"Consulting detective," John amended. "He's quite successful too, I haven't seen him fail yet," he added with a touch of pride. If he had to be dragged around the world by a madman, at least it was a madman with a perfect track record.

The writer - Rick Castle, as he'd introduced himself earlier - turned away from the computer screen.

"I've been reading your blog posts," he said. "Are those all reports of real cases?"

"Yes," John said. "I've been writing them down ever since I moved in with him."

"Awesome!" Castle repeated. He glanced wistfully at the computer screen. "I don't suppose you could tell me about all the details you've written out?" he said, pointing to a page that was almost completely censored.

John shook his head. "Sorry," he replied. "I didn't really think anyone would read them, I wrote them for myself."

"They're not bad," Castle said. "The one about the Chinese mafia and the secret code was a great read."

"Was it?" Ryan asked, moving his chair to read over Castle's shoulder.

John had been too involved in the case to treat it as just a story (there had been people who died!) but it was still flattering to hear his blog posts praised by a world-famous writer. Even though John had only read a couple of Derek Storm books and hadn't thought much of them.

"Have you ever thought of publishing a book?" Castle asked him.

John had, once or twice, but he doubted Sherlock would take kindly to the idea. He didn't even like it when newspapers reported his exploits, preferring to act behind the scenes. After Moriarty, John couldn't really blame him for it.

"No," he said instead. "I'm not a writer, really, this is just a hobby for me."

"The writing could be polished a bit," Castle admitted. "But you'd be surprised at the wonders that an editor can do. As long as you don't marry her," he added after some thought.

John took a sip of his coffee and tried not to spit it out. There was so much cream and sugar that it was hard to taste the coffee, though from previous experience maybe that was a blessing.

"And you'd need to find a better main character," Castle added. "Your friend wouldn't make a good lead if all he does is sit around and think. The readers want a hero, someone ready to fight off suspects and gets in car chases..."

"Believe me, we've been in plenty of chases," John replied in a small voice.

Castle clapped enthusiastically. "Oooh, do tell me," he said. "One writer to the next. I could add you two to my next book."

John glanced to Sherlock who looked deep in thought. He would be busy on the case for another while.

"All right," he said. "There was this case that he got right after we started living together..."

It wasn't as if anybody would ever be interested in reading about Sherlock Holmes' adventures.


End file.
